Households, industrial and agricultural processes, municipality operations and food and feed processing generate waste fractions and by-products containing polymeric and/or monomeric sugars, e.g. in the form of glucose, starch, cellulose and hemicellulose. Today the majority of waste fractions generated in the households and industry are either deposited or incinerated. Deposition of waste fractions is connected with various environmental, health and logistic problems and is therefore becoming increasingly restricted in many countries, e.g. EU countries. The major alternative treatment is incineration of the waste. In countries with widely distributed district heating systems, e.g. Denmark, waste-to-energy plants converting the combustible waste to e.g. electricity and district heating, provide a quite good utilisation of the energy content of the waste. However, for the majority of countries without district heating systems pure power generation of the waste is provided with a very low efficiency.
Furthermore, incineration of waste does cause a lot of new environmental problems, mainly NOx—, SO2— and dioxin-emissions. Significant investment in flue gas cleaning systems can reduce, but not eliminate such problems. Furthermore approximately 20% of the waste are non-combustible and are deposited as slag and fly ash. Nowadays parts of the slag are utilised as foundation materials in construction works, but future restrictions within such areas must be expected, mainly due to the fact that high content of heavy metal in the slag, could impede such an utilisation.
Thus, the vast amount of waste generated by modern society is becoming a problem with no obvious solutions. The local communities, industries as well as the society in general have considerable interest in developing processes for converting waste fractions in an environmentally friendly manner into materials of a higher value. Thus, by way of example various waste fractions could potentially be converted into bioethanol or other biochemicals by use of microorganisms and/or hydrolytic enzymes.
In general the key process steps in the production of bioethanol or other useful fermentation products from poly-, di- and monosaccharide containing waste fractions can be divided into five main steps, each step processed in one or more separate vessels:                Sorting and grinding of waste fractions        Pre-treatment        Hydrolysis        Fermentation        Product, e.g. ethanol, recovery        
The present invention surprisingly enables the steps of pre-treatment and hydrolysis to be performed in one and the same vessel in either a batch, semi-batch or continuous process and processes of the present invention do not as such rely on any previous sorting and grinding of the waste fractions. Furthermore, the fermentation, sorting and product, e.g. ethanol, recovery can also be performed in same vessel.
Sorting and Grinding of Waste Fractions
In process descriptions involving fermentation of non-homogeneous waste fractions, the first step is normally a very complicated process of sorting and grinding systems for the waste. The purpose is to gain an organic slurry capable of being processed in stirred tanks. The rest fractions should be recyclable to the widest extent. An example of such system is described in U.S. Pat. No. 4,094,740 A.
Pre-Treatment
Pre-treatment in general is required if subsequent hydrolysis (e.g. enzymatic hydrolysis) of the polysaccharides requires the break down of an otherwise protecting structure (e.g. lignin) of plant materials. Several pre-treatment techniques are known within e.g. the field of bioethanol production. Pre-treatment-processes may be based on e.g. acidic hydrolysis, steam explosion, oxidation, extraction with alkali or ethanol etc. A common feature of the pre-treatment techniques is that combined with the action of possible added reactants they take advantage of the softening and loosening of plant materials that occurs at temperatures above 100° C., i.e. a process requiring the application of pressure.
Hydrolysis
Following the pre-treatment and optionally the mechanical pre-treatment, the next step in the utilisation of mono- and/or polysaccharide-containing waste fractions for production of bioethanol or other biochemicals is hydrolysis of the liberated starch, cellulose and hemicellulose into fermentable sugars. If done enzymatically it requires a large number of different enzymes with different modes of action. The enzymes can be added externally or microorganisms growing on the biomass may provide them.
Cellulose is hydrolysed into glucose by the carbohydrolotic cellulases. The prevalent understanding of the cellulolytic system divides the cellulases into three classes; exo-1,4-β-D-glucanases or cellobiohydrolases (CBH) (EC 3.2.1.91), which cleave off cellobiose units from the ends of cellulose chains; endo-1,4-β-D-glucanases (EG) (EC 3.2.1.4), which hydrolyse internal β-1,4-glucosidic bonds randomly in the cellulose chain; 1,4-β-D-glucosidase (EC 3.2.1.21), which hydrolyses cellobiose to glucose and also cleaves off glucose units from cellooligosaccharides.
The different sugars in hemicellulose are liberated by the hemicellulases. The hemicellulytic system is more complex than the cellulolytic system due to the heterologous nature of hemicellulose. The system involves among others endo-1,4-β-D-xylanases (EC 3.2.1.8), which hydrolyse internal bonds in the xylan chain; 1,4-β-D-xylosidases (EC 3.2.1.37), which attack xylooligosaccharides from the non-reducing end and liberate xylose; endo-1,4-β-D-mannanases (EC 3.2.1.78), which cleave internal bonds; 1,4-β-D-mannosidases (EC 3.2.1.25), which cleave mannooligosaccharides to mannose. The side groups are removed by a number of enzymes; α-D-galactosidases (EC 3.2.1.22), α-L-arabinofuranosidases (EC 3.2.1.55), α-D-glucuronidases (EC 3.2.1.138), cinnamoyl esterases (EC 3.1.1), acetyl xylan esterases (EC 3.1.1.6) and feruloyl esterases (EC 3.1.1.73).
The most important enzymes for use in hydrolysis of a polysaccharide such as starch are alpha-amylases (1,4-α-D-glucan glucanohydrolases, (EC 3.2.1.1). These are endo-acting hydrolases which cleave 1,4-α-D-glucosidic bonds and can bypass but cannot hydrolyse 1,6-alpha-D-glucosidic branchpoints. However, also exo-acting glucoamylases such as beta-amylase (EC 3.2.1.2) and pullulanase (EC 3.2.1.41) can be used for starch hydrolysis. The result of starch hydrolysis is primarily glucose, maltose, maltotriose, α-dextrin and varying amounts of oligosaccharides. When a starch-based hydrolysate is used for fermentation addition of proteolytic enzymes can be advantageous. Such enzymes may prevent flocculation of the microorganism and may generate amino acids available to the microorganism.
In combination with pre-treatment and enzymatic hydrolysis of lignocellulosic biomass, it has been found that the use of oxidative enzymes can have a positive effect on the overall hydrolysis as well as the viability of the microorganisms employed for e.g. subsequent fermentation. The reason for this effect is the oxidative crosslinking of lignins and other phenolic inhibitors as caused by the oxidative enzymes. Typically laccase (EC 1.10.3.2) or peroxidase (EC 1.11.1.7) are employed either externally or by incorporation of a laccase gene in the applied microorganism.
Fermentation
The fermentation process used in relation to the production of bioethanol or other useful fermentation products from waste fractions containing mono- and/or polysaccharides is basically a biochemical reaction that breaks down complex organic molecules such as mono- and or polysaccharides, into simpler constituents such as ethanol, carbon dioxide, and water, through the action of yeast (standard, cultivated or manipulated) and/or bacteria or any other microorganism capable of producing ethanol or other specific chemicals from the present hexoses and pentoses.
Product, e.g., Ethanol, Recovery
Recovery of the product, e.g. ethanol, from the fermentation beer is a standard process normally divided into three main processes; Beer stripping, where the solids are separated from the ethanol/water solution, rectification, where the ethanol is recovered from the watery solution and dehydration, where the last water is removed from the ethanol.
The recovery processes will not be described further, as the processes are very similar to standard distillation systems used e.g. in the starch and sugar based ethanol production industry.
Processes Showing Similarities to the Present Invention
Viewed separately pre-treatment and several of the subsequent process steps leading to the production of bioethanol from organic waste fractions have previously been described.
U.S. Pat. No. 4,342,830A describes a process for thermal pre-treatment of organic matter such as commercial, industrial, agricultural, household and restaurant waste in drum type mixer, with an inner perforated rotated drum and an outer stationary drum. The vessel is under pressure and steam is added to soften the organic matter of the waste. By momentary depressurisation of the outer drum, the softened organic matter is forced through the perforations of the inner drum, and a sorting of organic and inorganic matters is thereby performed. Hence, the waste is intensively grinded before further hydrolysis. The organic matter can subsequently be used for several purposes including ethanol production, however, neither these processes nor the hydrolysis is described as taking place in the same vessel as the pre-treatment.
U.S. Pat. No. 4,093,516A describes mechanical pre-treatment, thermal pre-treatment, chemical hydrolysis and saccharification, fermentation and recovery of ethanol, however, the method is based on waste fractions with a low dry matter content such as liquefied municipal waste or sewage.
CZ9602835A3 describes a process for production of ethanol based on lignocellulosic and starch containing materials. The materials are hydrolysed in a thermal pressure vessel of the drum type. Enzymatic hydrolysis of the remaining lignocellulose is performed in a different vessel and the resulting mash is transferred to still another fermentation vessel, hence, use of a single vessel for pre-treatment and hydrolysis is not suggested in this document.
U.S. Pat. No. 4,094,740A describes a process comprising a 15-step waste sorting system based on the wet-fractionation system for the production of ethanol from municipal solid waste. The organic waste is subsequently ground and hydrolysed by pressurised acid hydrolysis for e.g. subsequent ethanol production.
U.S. Pat. No. 5,637,502 describes a process for utilising municipal solid waste in ethanol production. The process operates with pulped waste with a dry matter content below 20%. The waste is heated under pressure in a stirred tank. The hydrolysis is performed with enzymes and the glucose produced is continuously recovered through a five-step sorting unit.
Differences Between Present Invention and Known Technologies
Thus, there is a need for a simplified process which can handle waste fractions with a high dry matter content and which also allows efficient treatment of only partly-organic waste fractions containing large particles of also non-organic origin. The present invention provides a process capable of handling un-sorted waste fractions directly and thereby avoids the use of large, costly and environmentally problematic sorting and grinding systems. For municipal solid waste, sorting at the source, or alternatively some kind of central sorting of organic and inorganic fractions is believed to enhance performance of processes according to the present invention. For other fractions a rough shredding of the waste to open up bags and reduce volume is believed to enhance performance of processes according to the present invention.
The pre-treatment in a process according to the present invention is performed at atmospheric pressure which reduces the energy cost, the equipment cost and the mechanical difficulties significantly. Further more no chemical addition is needed in the pre-treatment. Therefore the present invention is directed at waste fractions where the polysaccharides mainly are sugars, starch or already pre-treated cellulose as paper, cardboard or similar. Hereby the costs for enzymes also are kept low, as amylases in general are cheaper than cellulases. The aim of this process is to do a low-priced extraction of monosaccharides. Unconverted lignocellulosics can possibly be sorted out after the fermentation and used for instance in a process with high pressure pre-treatment.
The hydrolysis is in the present invention performed enzymatically without previous detoxification of the mash. Furthermore the hydrolysis is carried out on a mash with dry matter content above 20% w/w.
The fermentation in this invention is also performed without any prior detoxification. Chemical addition is only needed for pH-adjustment. Optionally the liquefied waste fraction could be lead to a standard fermenting vessel for further saccharification and fermentation.
The next process step of the production of bioethanol or other useful fermentation products optionally includes the sorting of the fermented or fermentable waste fractions from the non-fermentable solids. This step can be performed in the same vessels used for pre-treatment, hydrolysis and fermentation. By utilising the fact, that the hydrolysis liquefies the fermentable parts of the waste while the non-fermentable solids remain in the solid phase, a sorting can e.g. be performed by a sieve system incorporated in or external to the vessel.
Furthermore it is possible to perform the product, e.g. ethanol, recovery in one and same vessel. This can either be as a total recovery of e.g. ethanol from the fermented mash accomplished by heating and/or vacuum, or it can be a recovery of e.g. ethanol from the none-fermented solids remaining after the sorting.
Within the technical field of e.g. bioethanol production it has, so far, not been possible to perform pre-treatment and hydrolysis in one single vessel utilising the principle of free fall mixing, and even less to perform the total process from pre-treatment to product recovery in one vessel.
The principle of gravity mixing can be applied for all kinds of waste fractions even with high viscosity or presence of large entangling particles with low energy inputs and allows easy scale up. Another object of the present invention is to reduce energy input in pre-treatment by using non-pressurised pre-treatment. It has surprisingly been found, that treatment of non-shredded, partly-organic waste fractions with a dry matter content above 20% according to the present invention, result in enzymatic hydrolysis of more than 50% of the cellulose, hemicellulose and starch originally present in the waste into cellobiose, glucose and xylose. Furthermore, an ethanol content above 4 vol. % is obtainable without adding other fermentable raw materials.